Cardiac rhythm management devices are implantable devices that provide electrical stimulation to selected chambers of the heart in order to treat disorders of cardiac rhythm. A pacemaker, for example, is a cardiac rhythm management device that paces the heart with timed pacing pulses. The most common condition for which pacemakers have been used is in the treatment of bradycardia, where the ventricular rate is too slow. Atrio-ventricular conduction defects (i.e., AV block) that are permanent or intermittent and sick sinus syndrome represent the most common causes of bradycardia for which permanent pacing may be indicated. If functioning properly, the pacemaker makes up for the heart's inability to pace itself at an appropriate rhythm in order to meet metabolic demand by enforcing a minimum heart rate and/or artificially restoring AV conduction. Pacing therapy may also be used in treatment of cardiac conduction disorders in order to improve the coordination of cardiac contractions, termed cardiac resynchronization therapy. Other cardiac rhythm management devices are designed to detect atrial and/or ventricular tachyarrhythmias and deliver electrical stimulation in order to terminate the tachyarrhythmia in the form of a cardioversion/defibrillation shock or anti-tachycardia pacing. Certain combination devices may incorporate all of the above functionalities. Any device with a pacing functionality will be referred to herein simply as a pacemaker regardless of other functions it may be capable of performing.
Cardiac rhythm management devices such as described above monitor the electrical activity of the heart via one or more sensing channels so that pacing pulses or defibrillation shocks can be delivered appropriately. Such sensing channels usually include implanted leads which have electrodes disposed intravascularly near the heart, which leads may also be used for delivering pacing pulses or defibrillation shocks. The signals generated from such sensing channels are referred to as intra-cardiac electrograms and reflect the time course of depolarization and repolarization as the heart beats, similar to a surface electrocardiogram (ECG). A problem that arises when pacemakers use intra-cardiac electrogram signals to detect intrinsic activity, however, is that pacing pulses delivered by the pacemaker interfere with the electrogram signal. The usual method by which a pacemaker deals with this problem is to temporarily disable a sensing channel for a specified time interval when a pacing pulse is delivered, referred to as a refractory period. The refractory period typically includes a blanking interval, during which time the sense amplifiers are disabled to prevent their saturation by the pacing pulse, as well as additional time in order to avoid interpreting a pacing pulse or an after-potential as an intrinsic beat. The longer a sensing channel is rendered refractory, however, the more compromised is its ability to detect tachyarrhythmias. Long refractory periods also make it more difficult to utilize a sensing channel to detect the electrical activity of the heart resulting from a pacing pulse, referred to as an evoked response, in order to determine if the pacing pulse has captured the heart. The present disclosure relates to a means for improving this situation.